1. Field of Invention
The invention relates to a method and control circuit for, in general, starting a gas-discharge lamp and, in particular, operating the lamp in a transition from a “deactivated” state without an electric arc to a stable “light-generating” state.
2. Description of Related Art
A method for operating a gas-discharge lamp in a transition from a “deactivated” state without an electric arc to a stable “light-generating” state and a corresponding control circuit are known per se. With the operation of a gas-discharge lamp—in particular, one to be used in gas-discharge lamps of lighting apparatuses designed for motor vehicles used on roads—an electric arc between two electrodes is generated in a glass bulb filled with gas. In the transition from the “inactive” state without an electric arc to a stable “light-generating” state, numerous phases can be distinguished—designated as the “ignition,” “acquisition,” and “start-up.” At this point, the normal operating state with a stably burning electric arc follows.
For the ignition, first, an ignition-voltage impulse is applied to the electrodes. The ignition-voltage impulse is very short and leads to an ionization of gas particles in the electric field between the electrodes. The extent of the impulse-like ignition voltage for typical commercial gas-discharge lamps for motor vehicle headlamps is between 20 kV and 30 kV.
In a phase designated as the “acquisition” energy stored in a booster capacitor is used to subsequently accelerate the ionized gas particles to the extent that, by impact ionization, snowballing charge breakdown is established between the electrodes, which ignites the electric arc, and the arc is sustained.
In doing so, the voltage of the booster capacitor (previously charged to about 400 V) decreases to a lamp voltage that can be adjusted for stable operation. For lamps containing Hg, this is about 80 V. Lamps not containing Hg are operated with a lamp voltage of 43 V. It is generally accepted that the lamp voltage can be between 30 V and 120 V, depending on the design of the lamp. The “acquisition” phase lasts, for example, for a few hundred microseconds.
Following the acquisition, the starting-up of the gas-discharge lamp occurs with a temporary “direct current” operation, which serves to heat the electrodes quickly. A typical duration, of a “direct current” operation lasts for 50 milliseconds. Normally, a second “direct current” phase of the same length follows a first “direct current” phase and has a reversed polarity.
Subsequently, the gas-discharge lamp is operated in the normal operating state with an alternating current having a frequency of 250 Hz to 800 Hz—in particular, at about 400 Hz—and a value for the lamp voltage between the two electrodes dependent on the design of the lamp, which lies between 30 V and 120 V. The operation with alternating current serves to establish a limiting of a loss of contact material in the electrode.
In this context, the invention concerns the discharge of the booster capacitor in the phase designated as the “acquisition” with which the energy is made available for the snowballing breakdown resulting from a charge acceleration and impact ionization. The term “acquisition” indicates the transition to the electric arc.
The acquisition behavior of gas-discharge lamps depends on, among other things, the quantity of energy made available during the “acquisition” phase. For a reliably reproducible acquisition (i.e., reliably resulting buildup of the electric arc), it is necessary that the time period of the current flow of the discharge current from the booster capacitor exceeds a predetermined minimum value and the current of the discharge current neither falls below a predetermined minimum value during this time period nor exceeds a predetermined maximum value.
The limiting to a maximum value serves to protect the gas-discharge lamp and circuit components conducting the discharge current from an unacceptable high current load. Not falling below the minimum value and time period, on the other hand, is necessary for preventing an extinguishing of the electric arc after the discharging of the booster capacitor.
In the transition from the “non-activated” state (without an electric arc) to a state of the gas-discharge lamp in which a stable light is generated, the booster capacitor is discharged by a current pathway in the “acquisition” phase following the ignition-voltage impulse that flows through the current flowing through the gas-discharge lamp and in which an inductor having at least one circuit is disposed in series. With the related art, the inductor is formed from the secondary inductor of an ignition transformer that provides the ignition impulse, and the discharge current flows through a discharge resistor connected in series to the booster capacitor. The booster capacitor serves, thereby, as an energy source in a series connection arising from the discharge resistor and gas-discharge lamp. The discharge resistor increases, thereby, the resistance in the discharge-current circuit, which increases the discharge time period and reduces the extent of the discharge current.
With a circuit of this type, the energy stored in the booster capacitor is distributed during the discharge in relation to the impedances from the discharge resistor and gas-discharge lamp to the discharge resistor and gas-discharge lamp. The portion of energy for the discharge resistor is transformed to heat therein and is, thereby, made unavailable for the build-up and sustaining of the electric arc. With a cold start of the gas-discharge lamp, the energy portion has a lower value in comparison with the discharge resistor such that the major portion of the stored energy in the discharge resistor is converted to heat such that it serves no purpose.
A suitable dimensioning of the discharge resistor is characterized as being technically and economically difficult primarily by an increased ambient temperature of the resistors from 150° C.
Thus, there is a need in the related art for a method and control circuit of the respective types specified above that ensure that the gas-discharge lamp (independently of its state—in particular, its type, age, manufacturer, and variability of characteristics due to manufacturing conditions) is provided with the energy necessary for a successful acquisition, whereby the specified technical and economical difficulties associated with the dimensioning of one or more discharge resistors do not occur.